Dec 18 (Reuters) -

Activist investor Carl Icahn said on Monday that he plans to oust directors at Illumina, signalling plans to mount a second board challenge at the gene-sequencing company only months after shareholders elected one of his director candidates.

Icahn disclosed his plans without offering details in a letter to other shareholders less than 24 hours after Illumina said it will divest blood test maker Grail.

The billionaire investor long criticized Illumina's Grail acquisition and made it a centrepiece of his previous proxy fight where Andrew Teno, a portfolio manager working for Icahn, won a seat.

On Monday Icahn cheered the decision but said the job at Illumina has not been fully completed.

"Our third goal is to remove these legacy conflicted directors," Icahn wrote in a letter that was made public in a regulatory filing. His first two goals were to push out former CEO Francis deSouza, who resigned in June, and get Illumina to divest Grail, the letter said.

Icahn said Illumina's legacy directors cannot be trusted to oversee the disposal of Grail. "It would be a great mistake to allow the legacy conflicted directors to influence Illumina given their history of reckless decision-making and value destruction," the letter said.

Illumina's stock price climbed 1% in early trading to $128.38.

(With additional reporting by Khushi Mandowara in Bengaluru; Editing by Arun Koyyur and Andrea Ricci)